Magneto resistive heads (or MR heads) are used in data storage devices, such as disk drives, tape drives, removable and flexible media drives, etc., where data is stored on the data storage media (or disks) in the form of small, permanently magnetized regions. The magnetized regions produce magnetic fields due to divergence in the magnetization. As the MR head passes over the disk and reads the data stored thereon, the magnetic field from the permanently magnetized regions modulates the resistance of the MR head. A biasing signal, either current or voltage, typically is supplied to bias the magnetization of the MR head to a quiescent condition of optimum sensitivity and minimum distortion. If a biasing current is used, it is applied through the MR head. If a biasing voltage is used, it is applied across the MR head. A variation in the current or voltage can be sensed to read data values as the head passes over the disk.
Each mode has different advantages and limitations. For example, current biasing has fewer noise problems and is usually adequately sensitive for performing read operations. However, current biasing, particularly with high current amplitude values, can cause premature aging of the MR head. Voltage biasing can be straight forward from a circuit standpoint, and is subject to noise issues and does not provide the sensitivity level desired in some applications.
Today, the electronic biasing circuitry, such as preamplifiers, supporting the MR heads in most conventional data storage devices are designed for only one biasing mode, either current or voltage. Similarly, the sensing electronic circuitry supporting the MR heads in most conventional data storage devices is limited to only one biasing scheme. Depending on the choice in the biasing and sensing modes, four biasing and sensing schemes can be defined: (1) current biasing/current sensing, (2) current biasing/voltage sensing, (3) voltage biasing/voltage sensing, and (4) voltage biasing/current sensing. The manufacturers of the disk drive will test the head and select an optimum scheme for each drive and application. The combination of current biasing/voltage sensing is not often used, so today disk makers select from one of three different preamplifiers biasing and sensing schemes when making a disk drive. As a consequence, each preamplifier is very specific and limited in its application, and must be intimately adjusted at the time of manufacture for the particular data storage device.